This invention relates to a zirconia ceramic (sintered body) excellent in durability at high temperatures and usable as mechanical parts and a process for producing the same.
Ceramics containing zivconium oxide as a main component have been used as refractory heat insulation materials, since these ceramics are low in thermal conductivity and difficult in thermal deformation. Recently one or more stabilizing agents such as yttrium oxide, calcium oxide and magnesium oxide in small amounts are solid solubilized in zirconium oxide for by far improving flexural strength, hardness so as to be used as cutting tools. But in order to obtain ceramics with high strength and high hardness, it is necessary to use very fine raw material powders produced by gas-phase or liquid-phase reactions, and to limit the sintering temperature to a narrow range. Such limited conditions are naturally necessary since the size and kind of crystals included in sintered bodies should be limited. Thus, the production of large-sized products is very difficult and even if produced, the resulting products are very expensive, which results in limiting their use.
Further, the thus produced sintered bodies are thermally unstable. For example, change in crystal system and lowering in strength take place during a long use at 200.degree.-500.degree. C. in the case of a zirconium oxide sintered body solid solubilized with yttrium oxide and at near 1000.degree. C. in the case of a zirconium oxide sintered body solid solubilized with calcium oxide, which results in limiting the use at high temperatures. Particularly in the case of industrial parts, when they are subject to precise cutting workings such as diamond grinding, and the like, the worked side brings about great strain and easily brings about crystal change due to the heat produced, which results in lowering of strength. On the other hand, when a large amount of stabilizing agent is solid solubilized, thermal stability can be obtained but the strength is lowered, which results in making it unsuitable to use these sintered bodies as mechanical parts.